The present invention relates to an apparatus for multiplexing digital signals, and more particularly to a multiplexing apparatus which is well suited to a subscriber line terminal apparatus wherein digital signals from a plurality of terminal equipments are multiplexed and then sent to a subscriber line.
When transmitting a plurality of digital signals each corresponding to different kinds of information, such as data, voice and pictures, by the use of a single subscriber line, prior practice is that a TDM (time division multiplexed) frame which is transmitted on the subscriber line is divided into channels corresponding to the respective information items. A multiplexing apparatus which multiplexes the information items into the corresponding channels of such a TDM frame, is discussed in "Architecture and Technology of the Local Exchange Network in the Broadband ISDN" by H. Bauch et al., issls 86 Proceedings, pp. 29-34. This apparatus is shown in FIG. 2, and the format of the TDM frame in FIG. 3. As indicated in this example, a subscriber line 23 of fixed capacities (in FIG. 2, a downstream capacity of 614 Mb/s and an upstream capacity of 154 Mb/s) has been offered to each subscriber, and information signals from terminal equipment have been multiplexed in the respective channels 32, 33-33"' and 34-34"' of the predetermined frame format. The number 31 denotes a frame alignment signal.
In the prior art, the subscriber must use the subscriber line of the fixed transmission capacities irrespective of required transmission capacities. Moreover, the sorts and number of the terminal equipment to be used are limited by the channel format of the multiplexed frame 30, as shown in FIG. 3.
In actuality, the transmission capacities which are required by the individual subscribers are of infinite variety, and even the same subscriber might expand small capacities into large ones. In the prior art, such multifarious requests cannot be met economically.
In regard to the terminal equipment to be used, some subscribers favor ones of small capacities in large numbers, while others might desire ones of large capacities. Furthermore, even the same subscriber might use both the terminal equipment of small and large capacities separately according to times. The prior art cannot economically accommodate these flexible requirements either.